U.S. patent application number 13/621313 was filed with the patent office on 2013-03-28 for method and system for determining the position of an aircraft during its approach to a landing runway.
This patent application is currently assigned to ECOLE NATIONALE DE L'AVIATION CIVILE (E.N.A.C.). The applicant listed for this patent is Airbus Operations S.A.S., Ecole Nationale de L'Aviation Civile (E.N.A.C.). Invention is credited to Laurent Azoulai, Christophe Macabiau, Jean Muller, Pierre Neri.
Application Number | 20130079958 13/621313 |
Document ID | / |
Family ID | 46851340 |
Filed Date | 2013-03-28 |
United States Patent
Application |
20130079958 |
Kind Code |
A1 |
Neri; Pierre ; et
al. |
March 28, 2013 |
METHOD AND SYSTEM FOR DETERMINING THE POSITION OF AN AIRCRAFT
DURING ITS APPROACH TO A LANDING RUNWAY
Abstract
A method for guiding an aircraft during its final approach to a
landing runway, whereby the aircraft is guided during its approach
by aircraft position information obtained from an GNSS satellite
navigation system, wherein: prior to the start of the final
approach a first time t.sub.FAF is determined corresponding with
the start of said final approach and a second time t.sub.TD
corresponding with the landing of the aircraft on said runway, then
a set of satellites is determined of the satellite navigation
system for excluding from the calculation of said aircraft position
information during at least a part of the time interval comprised
between said first and second times; and during the final approach,
the aircraft position information is determined while excluding the
information corresponding with all the satellites of said set of
satellites and the aircraft is guided along its final approach path
by said position information.
Inventors: |
Neri; Pierre;
(Tournefeuille, FR) ; Azoulai; Laurent;
(Mondonville, FR) ; Muller; Jean; (Tournefeuille,
FR) ; Macabiau; Christophe; (Montauban, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Airbus Operations S.A.S.;
Ecole Nationale de L'Aviation Civile (E.N.A.C.); |
Toulouse Cedex 9
Toulouse |
|
FR
FR |
|
|
Assignee: |
ECOLE NATIONALE DE L'AVIATION
CIVILE (E.N.A.C.)
Toulouse
FR
AIRBUS OPERATIONS S.A.S.
Toulouse Cedex 9
FR
|
Family ID: |
46851340 |
Appl. No.: |
13/621313 |
Filed: |
September 17, 2012 |
Current U.S.
Class: |
701/16 |
Current CPC
Class: |
G01S 19/15 20130101;
G01S 19/28 20130101; G01S 19/20 20130101 |
Class at
Publication: |
701/16 |
International
Class: |
G06F 19/00 20110101
G06F019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 22, 2011 |
FR |
11 58446 |
Claims
1. A method for guiding an aircraft during an approach in
anticipation of landing on a runway, whereby said approach is
selected in advance and the aircraft is guided during said approach
by means of aircraft position information obtained from at least
one GNSS satellite navigation system, wherein: prior to the start
of the final approach corresponding with said approach, the
following is determined in automatic manner: a) a first time
t.sub.FAF corresponding with the start of said final approach; b) a
second time tTD corresponding with the landing of the aircraft on
the runway; and c) a set of satellites of said satellite navigation
system that will be excluded from the calculation of said aircraft
position information during at least one portion of the time
interval comprised between said first and second times; during said
final approach, the following steps are performed in automatic and
iterative manner: d) said aircraft position information is
determined by excluding the information coming from all satellites
of said satellite set; and e) the aircraft is guided along its
final approach path by means of said position information.
2. The guidance method according to claim 1, wherein the set of
satellites determined in step c) corresponds with the satellites
that will be setting, relative to the aircraft, during said time
interval.
3. The guidance method according to claim 1, wherein in step d),
for the determination of the aircraft position information, the
satellites that will be rising, relative to the aircraft, during
said time interval, are excluded.
4. The guidance method according to claim 2, wherein to determine
if a satellite Si will be setting, relative to the aircraft, during
said time interval, the two following equations are used: a =
.theta. i ( k ) - .theta. i ( k - 1 ) t ( k ) - t ( k - 1 )
##EQU00005## where: t(k) is the actual time; t(k-1) is a time
preceding the actual time .theta..sup.i(k) is the elevation angle
of the satellite Si relative to the aircraft at time t(k);
.theta..sup.i(k-1) is the elevation angle of the satellite Si
relative to the aircraft at time t(k-1); and t set i = .theta. mask
- .theta. i ( k ) a + t ( k ) ##EQU00006## where: t.sup.i.sub.set
is the setting time of the satellite Si; .theta.mask is the masking
angle selected for the satellite navigation system to which
satellite Si belongs; and if time t.sup.i.sub.set belongs to time
interval [t.sub.FAF; t.sub.TD] it is assumed that satellite Si will
be setting during said time interval.
5. The guidance method according to claim 1, wherein said time
t.sub.FAF is determined by calculating an estimated time of arrival
ETA.sub.FAF of the aircraft in the start position of the final
approach and said second time t.sub.TD is determined by calculating
an estimated time of arrival ETA.sub.TD of the aircraft in the
landing position on the runway.
6. The guidance method according to claim 5, wherein said first and
second times are calculated by means of the following equations:
t.sub.FAF=ETA.sub.FAF-.DELTA.t.sub.FAF
t.sub.TD=ETA.sub.TD+.DELTA.t.sub.TD where .DELTA.t.sub.FAF and
.DELTA.t.sub.TD are predetermined time margins.
7. The guidance method according to claim 1, wherein said aircraft
position information is obtained by combining information
originating from at least one GNSS satellite navigation system and
at least one augmentation system.
8. The guidance method according to claim 7, wherein said at least
one augmentation system is a SBAS or ABAS or GBAS type system.
9. The guidance method according to claim 1, wherein said aircraft
position information is obtained by using a plurality of GNSS
satellite navigation systems.
10. A guidance system for an aircraft, the guidance system
comprising: means for selecting an approach in anticipation of a
landing on a runway; means configured for receiving signals from a
GNSS satellite navigation system; a processing unit for determining
the aircraft position information starting from said signals
received from the GNSS satellite navigation system; an aircraft
guidance means configured for receiving from the processing unit
said aircraft position information and for producing signals for
guiding the aircraft along a final approach path corresponding with
the selected approach, in function of said position information,
means for determining, prior to the start of the final approach, a
first time t.sub.FAF corresponding with the start of said final
approach and a second time t.sub.TD corresponding with the landing
of the aircraft on said runway; and means for determining, prior to
the start of the final approach, a set of satellites of said
satellite navigation system that will be excluded from the
calculation of said aircraft position information during at least a
part of the time interval comprised between said first and second
times; wherein, said processing unit is configured to determine the
aircraft position information, during the final approach, while
excluding the information corresponding with the satellites of said
set of satellites.
11. The guidance system for an aircraft according to claim 10,
wherein the processing unit is configured for receiving information
from an augmentation system and for determining the aircraft
position information starting at least from signals received from
the GNSS satellite navigation system and said information received
from an augmentation system.
12. The guidance system for an aircraft according to claim 10,
further comprising means (32) configured for receiving signals from
a plurality of GNSS satellite navigation systems, whereby the
processing unit is configured for determining the aircraft position
information starting from at least the signals received from said
plurality of GNSS satellite navigation systems.
13. An aircraft comprising a guidance system, said guidance system
comprising: means for selecting an approach in anticipation of a
landing on a runway; means configured for receiving signals from a
GNSS satellite navigation system; a processing unit for determining
the aircraft position information starting from said signals
received from the GNSS satellite navigation system; an aircraft
guidance means configured for receiving from the processing unit
said aircraft position information and for producing signals for
guiding the aircraft along a final approach path corresponding with
the selected approach, in function of said position information,
means for determining, prior to the start of the final approach, a
first time t.sub.FAF corresponding with the start of said final
approach and a second time t.sub.TD corresponding with the landing
of the aircraft on said runway; and means for determining, prior to
the start of the final approach, a set of satellites of said
satellite navigation system that will be excluded from the
calculation of said aircraft position information during at least a
part of the time interval comprised between said first and second
times; wherein, said processing unit is configured to determine the
aircraft position information, during the final approach, while
excluding the information corresponding with the satellites of said
set of satellites.
Description
[0001] The present invention relates to the domain of aircraft
navigation and more particularly to a method for determining the
calculated position of an aircraft, based on GNSS satellite
navigation ("Global Navigation Satellite System" in English), such
as GPS, during the final approach phase of a precision approach in
automatic mode, leading to an automatic landing also called
"autoland" in Anglo-Saxon terminology. The invention is more
particularly related to a method and device for determining
position information of the aircraft during said final approach, so
that the lateral and vertical deviation information supplied to the
automatic pilot and specifically to the guidance laws of precision
approach, will not result in jumps which could affect the
performance of the automatic approach and landing.
[0002] In the domain of civil aviation, landing assistance systems
exist allowing an aircraft to fly precision approach operations.
These systems provide an indisputable operational benefit by
guiding the aircraft in reliable manner, thanks in particular to
vertical guidance up to a decision height corresponding with
minimum heights, in general less than or equal to 200 feet
(approximately 60 meters), in function of the category (cat-I to
cat-III) of the intended precision approach.
[0003] These minimum heights are even zero for category Cat-IIIC
approaches. Some of said approaches can end in an entirely
automatic landing. The main existing landing assistance systems,
used for executing precision approaches, are ILS ("Instrument
landing System" in English) and MLS ("Microwave Landing System" in
English). These landing assistance systems are relying on one or
more ground stations dedicated to this function and on means,
aboard the aircraft, for receiving the signals emitted by said
ground stations.
[0004] Landing assistance systems also exist using aircraft
position information determined by means of a GNSS satellite
positioning system, in which said position is compared with a
reference path corresponding with the anticipated approach. The
precision, the integrity, the continuity and the availability of
the position information used by said landing assistance systems
can be improved by so-called augmentation techniques. These
techniques were defined in particular by the OACI ("International
Civil Aviation Organization"). It is for instance possible to use
GBAS type augmentation ("Ground based Augmentation System" in
English), as in the GLS landing assistance system ("Ground based
augmentation landing System" in English), in order to carry out
precision approaches.
[0005] One future objective is to be able to execute precision
approaches, which could include automatic landing, in any type of
airport, therefore also in airports not equipped with ground
stations, such as for instance the stations used for ILS or MLS
systems. For this purpose, it will be necessary to use landing
assistance systems using aircraft position information determined
starting from a GNSS type satellite positioning system, and
augmentation techniques which do not necessarily rely on ground
stations inside the airport. These augmentation techniques can be,
for instance, SBAS type techniques ("Satellite Based Augmentation
System" in English) or ABAS type techniques ("Airborne Based
Augmentation System" in English) defined by the OACI. This last
type of augmentation relies in particular on RAIM type techniques
("Receiver Autonomous Integrity Monitoring" in English) and/or AAIM
type techniques ("Aircraft Autonomous Integrity Monitoring" in
English). SBAS type augmentations can be implemented by means of
systems such as WAAS ("Wide Area Augmentation System" in English)
in the USA, or EGNOS ("European Geostationary Navigation Overlay
System" in English) in Europe.
[0006] In the approaches relying on landing assistance systems
using aircraft position information determined starting from a GNSS
type satellite positioning system, said position information is
calculated in 3 dimensions starting from distance measurements,
called pseudo distance between GNSS satellites and one or more GNSS
receivers on board of the aircraft. The performance and the
behavior in time of the GNSS position in 3 dimensions depend on
different error contributors linked to the satellite constellation,
the propagation effects of the GPS signal through the atmosphere
and the quality of the receiver on the one hand, and on the GNSS
constellation geometry on the other hand. For instance, in the case
of the existing GPS constellation, a user receiver sees satellites
rising and/or setting as result of their orbit around the earth,
whereby the orbit consists of one revolution in 23 hours and 56
minutes. A satellite can also be removed from the calculation of
the GPS based position because of a defect of said satellite
detected by the receiver thanks to an augmentation system such as
SBAS, GBAS or ABAS. This addition or removal of one or more
satellites in the calculation of the aircraft position, during the
approach, can create position jumps of a few meters. These position
jumps, which are acceptable in the context of approaches limited to
a decision height, for instance of 200 feet (approximately 60
meters), might not be acceptable for precision approaches with
lower decision height, in particular in case of automatic landing.
In fact, given the required high performance for guiding an
automatic landing, it is necessary to know with great precision (a
few meters) the position of the aircraft and a position jump could
be assimilated with a strong bias, which in some cases might not be
acceptable.
[0007] An aspect of the present invention is to remedy at least
some of the aforementioned disadvantages. It relates to a method
for guiding an aircraft in its the approach to a landing runway,
whereby said approach was selected in advance and during said
approach the aircraft is guided by means of aircraft position
information obtained from at least one GNSS satellite navigation
system,
[0008] This system is remarkable in that:
prior to the final approach corresponding with said approach, the
following occurs in automatic manner: [0009] a) a first time
t.sub.FAF is determined corresponding with the start of said final
approach; [0010] b) a second time t.sub.TD is determined
corresponding with the landing of the aircraft on said runway; and
[0011] c) a set of satellites is identified of said satellite
navigation system that will be excluded from the calculation of
said aircraft position information during at least a part of the
time interval comprised between said first and second times; during
said final approach, the following steps are carried out in
automatic and iterative mode: [0012] d) said aircraft position
information is determined by excluding information originating from
all satellites of said set; and [0013] e) the aircraft is guided
along its final path by means of said position information.
[0014] This method offers the advantage of avoiding jumps in the
value of said position due to the exclusion of a satellite relative
to the aircraft during the final approach, since the satellites
that are excluded during this final approach are not taken into
account, from the beginning of the final approach, for the
calculation of the aircraft position.
[0015] By preference, the satellite set determined in step c)
corresponds with the satellites that will be setting relative to
the aircraft, during said time interval, In this way, a jump in the
value of the position is avoided due to a satellite setting
relative to the aircraft during the final approach.
[0016] By preference, still, in step d), satellites that will rise
relative to the aircraft during said time interval, are also
excluded from the calculation of the aircraft position information.
In this way, a jump is avoided in the value of said position due to
a satellite rising relative to the aircraft during the final
approach.
[0017] In a particular implementation mode, the two following
equations are used in order to determine whether a satellite Si
will be setting relative to the aircraft, during said time
interval:
a = .theta. i ( k ) - .theta. i ( k - 1 ) t ( k ) - t ( k - 1 )
##EQU00001##
where: t(k) is the actual time; t(k-1) is a time preceding the
actual time .theta..sup.i(k) is the elevation angle of the
satellite Si relative to the aircraft at time t(k);
.theta..sup.i(k-1) is the elevation angle of the satellite Si
relative to the aircraft at time t(k-1); and
t set i = .theta. mask - .theta. i ( k ) a + t ( k )
##EQU00002##
where: t.sup.i .sub.set is the setting time of the satellite Si;
.theta..sub.mask is the masking angle selected for the satellite
navigation system to which satellite Si belongs; and if time
t.sup.i .sub.set belongs to time interval [t.sub.FAF; t.sub.TD] it
is assumed that satellite Si will be setting during said time
interval.
[0018] In advantageous manner, said first time t.sub.FAF is
determined by calculating the estimated time of arrival ETA.sub.FAF
of the aircraft in the start position of the final approach and
said second time t.sub.TD is determined by calculating the
estimated time of arrival ETA.sub.TD of the aircraft in the landing
position on the runway.
[0019] In this case, in advantageous manner, said first and second
times are calculated by using the following equations:
t.sub.FAF=ETA.sub.FAF-.DELTA.t.sub.FAF
t.sub.TD=ETA.sub.TD +.DELTA.t.sub.TD
where .DELTA.t.sub.FAF and .DELTA.t.sub.TD are predetermined time
margins.
[0020] In a preferred implementation mode, said aircraft position
information is obtained by combining information coming from at
least one GNSS satellite navigation system and at least one
augmentation system. Said at least one augmentation system can be
in particular a SBAS, ABAS or GBAS type system. The use of
information coming from an augmentation system improves the
integrity of the position information. With SBAS or ABAS type
systems, an approach can be carried out to an airport not equipped
with ground stations.
[0021] In advantageous manner, a plurality of GNSS satellite
navigation systems are used for obtaining said aircraft position
information. This offers the advantage of enabling the calculation
of the position information even if the number of not excluded
satellites belonging to the same satellite navigation system and
usable during the final approach is insufficient to calculate this
position information. Indeed, the use of satellites belonging to
other satellite navigation systems provides compensation for such
insufficiency. In addition, even if the number of satellites is
sufficient, it provides an opportunity to improve the precision,
the integrity and the continuity of said position information.
[0022] The invention relates also to an aircraft comprising a
system suitable for implementing the aforementioned guidance
method.
[0023] The invention relates also to a guidance system of an
aircraft comprising: [0024] means for selecting an approach in
anticipation of a landing on a runway; [0025] means suitable for
receiving signals from a GNSS satellite navigation system; [0026] a
processing unit for calculating aircraft position information
starting from said signals received from the GNSS satellite
navigation system; [0027] aircraft guidance means, suitable for
receiving from the processing unit said aircraft position
information and for producing aircraft guidance signals along a
final approach path corresponding with a selected approach, in
function of said position information.
[0028] This system is also remarkable in that, it comprises
furthermore: [0029] means for determining, prior to the start of
the final approach, a first time t.sub.FAF corresponding with the
start of said final approach and a second time t.sub.TD
corresponding with the landing of the aircraft on said runway.
[0030] means for determining, prior to the start of the final
approach, a set of satellites of said satellite navigation system
that will be excluded from the calculation of said aircraft
position information during at least a part of the time interval
between said first and second times; and in that, said processing
unit determines the aircraft position information, during the final
approach, by excluding the information originating from all the
satellites of said set of satellites.
[0031] In a preferred implementation mode, the processing unit is
furthermore suitable for receiving information from an augmentation
system and for determining the aircraft position information
starting from at least signals received from the GNSS satellite
navigation system and said information received from an
augmentation system
[0032] In advantageous manner, this guidance system comprises means
suitable for receiving signals from a plurality of GNSS satellite
navigation systems, whereby the processing unit determines the
aircraft position information starting from at least the signals
received from said plurality of GNSS satellite navigation
systems.
[0033] The invention relates also to an aircraft comprising the
aforementioned guidance system.
[0034] The invention will be better understood by reading the
following description and by examining the attached figures.
[0035] FIG. 1 is a block diagram of an aircraft guidance system
according to the invention.
[0036] FIG. 2 is a representation of an aircraft in its final
approach to the runway.
[0037] FIGS. 3 and 4 are block diagrams of sub-assemblies of the
aircraft guidance system according to the invention.
[0038] FIG. 5 shows the elevation angle of a satellite relative to
an aircraft.
[0039] FIG. 1 shows an aircraft guidance system according to an
embodiment of the invention. This guidance system comprises several
aircraft guidance processors, in particular an FMS type ("Flight
Management System" in English) flight management processor 10 and a
MMR multi-mode receiver 14 ("Multi Mode Receiver" in English). This
receiver 14 comprises in particular means 30 for receiving GNSS
signals, for instance GPS. It comprises also one or several
processors 15 supporting xLS functions available on the aircraft;
the term xLS indicates in general manner the different landing
assistance systems such as ILS, MLS, GLS . . .
[0040] In preferred manner, the FMS flight management processor 10
comprises a navigation information database 12. This database
contains specific information relative to runway approaches.
Furthermore, in known manner, the flight management processor 10 is
connected to a man-machine interface 16 which is for instance an
MFD type ("Multi Function Display" in English) or a MCDU type
interface ("Multipurpose Control and Display Unit" in English).
[0041] The guidance system comprises also a guidance processor 18,
for instance an AFS type processor ("Auto Flight System" in
English) suitable for processing guidance commands for the
aircraft, which can be used by the automatic pilot in automatic
guidance mode or by a flight director. This guidance processor is
connected to a man-machine interface 20, for instance an FCU type
interface ("Flight Control Unit" in English).
[0042] In preferred manner, although not indispensable for the
implementation of the invention, the guidance system can comprise
furthermore: [0043] a RMP type ("Radio management panel" in
English) radio command panel 22, which can be used by the pilot to
select an approach through approach mode and the frequency (or
channel) in case of defect of the MFD (or MCDU) 16 or in case of
defect of the FMS processor 10; [0044] an EFIS type ("Electronic
Flight Instrument" in English) processor 24 which corresponds with
an electronic flight instrument system displayed on the display
means of the aircraft; [0045] a FWS type ("Flight Warning System"
in English) warning system 26; and [0046] a DFDR ("Digital Flight
data Recorder" in English) recording processor 28 which corresponds
with a digital flight data recorder.
[0047] In traditional manner in aeronautics, the different
processors of the guidance system are by preference the object of
redundancy for reasons of operational safety. For instance, the
guidance system can comprise two or three FMS, two MMR, two or
three AFS, two FCU, two RMP, two EFIS, two FWS and two DFDR.
[0048] The guidance system can also be implemented by means of
specific processors for each function (FMS, MMR, FWS . . . ) in
general called LRU ("Line Replacement Unit" in English), or in the
form of an IMA type ("Integrated Modular Avionics" in English)
distributed modular architecture in which the different functions
are implemented in non specific processors communicating with each
other.
[0049] In anticipation of a landing of aircraft 1 on a runway 2
shown in FIG. 2, the aircraft pilot selects an approach to said
runway among the possible approaches for this runway, which are
stored in the navigation database 12 of the flight management
processor 10. The pilot can select this approach by means of the
MCDU Interface 16. This selection can be done either when entering
the flight plan in the FMS, or during a cruising phase of the
aircraft, prior to the approach. In the rest of the document, only
the case is considered in which the selected type of approach is
such that the guidance of the aircraft along the approach axis, in
particular along the final approach axis 3, takes place by using
aircraft position information obtained by means of at least one
GNSS satellite navigation system.
[0050] The final approach corresponds with the portion of the
approach between the FAF point ("Final Approach Fix" in English)
corresponding with the start of the final approach and the TD point
("Touch Down" in English) corresponding with the landing of the
aircraft on runway 2, specifically the touchdown point of the
wheels of the aircraft on the runway. The FAF point is in general a
predetermined point included in the navigation database 12.
[0051] According to an embodiment of the invention, prior to the
start of the final approach, the following information is
automatically determined: [0052] a first time t.sub.FAF
corresponding with the start of the final approach, in other words
the aircraft passing through point FAF; and [0053] a second time
T.sub.TD corresponding with the landing of the aircraft on the
runway, in point TD.
[0054] In preferred manner, the times T.sub.FAF and T.sub.TD are
determined by the FMS which calculates for this, in known manner,
an estimated time of arrival (called ETA, in other words "Estimated
Time of Arrival" in English) of the aircraft in points FAF and TD.
The FMS determines also the estimated times of arrival ETA.sub.FAF
and ETA.sub.TD corresponding respectively with time T.sub.FAF and
T.sub.TD.
[0055] In preferred manner, still, the predetermined time margins
.DELTA.t.sub.FAF and .DELTA.t.sub.m are taken into account relative
to the estimated times of arrival ETA.sub.FAF and ETA.sub.TD, which
eliminates the uncertainties linked to the estimation of said
arrival times. The time T.sub.FAF and T.sub.TD can then be
expressed by the following equations:
t.sub.FAF=ETA.sub.FAF.DELTA.t.sub.FAF
t.sub.TD=ETA.sub.TD+.DELTA.t.sub.TD
[0056] As an example, said predetermined time margins
.DELTA.t.sub.FAF and .DELTA.t.sub.TD can be selected in a time
interval between 1 second and 30 seconds, by preference 10 seconds,
in function of the accuracy of the estimated arrival times
ETA.sub.FAF and ETA.sub.TD.
[0057] Furthermore, always prior to the start of the final
approach, a set of satellites is identified of said satellite
navigation system that will be excluded from the calculation of
said aircraft position information during at least a part of the
time interval between said first and second times T.sub.FAF and
T.sub.TD. The satellites that will be excluded from said
calculation correspond in preferred manner with the satellites that
will be setting, relative to the aircraft, during said time
interval.
[0058] In an implementation example shown in FIG. 3, the values of
times T.sub.FAF and T.sub.TD are transmitted by the flight
management processor 10 to the GNSS receiver 30 integrated in the
multi-mode receiver MMR 14. The set of satellites of the satellite
navigation system that will be excluded from the calculation of the
aircraft position information is then determined by calculation
means integrated in said GNSS receiver 30. For instance, these
calculation means can be part of a position calculation unit
34.
[0059] In order to obtain the best possible estimate of the
satellites that will be setting during said time interval, it is
preferred to determine said satellite set a short time prior to the
start of the final approach phase, corresponding with the aircraft
passing through point FAF. For instance, the determination of said
set of satellites can be done at a time between the time interval
[T.sub.FAF-1minute; T.sub.FAF].
[0060] In a particular implementation mode of the invention, to
determine the set of satellites that will be setting relative to
the aircraft during the time interval between T.sub.FAF and
T.sub.TD, the following two equations are used for each satellite
Si of the satellite navigation system visible from the aircraft at
the time that said satellite set is determined:
a = .theta. i ( k ) - .theta. i ( k - 1 ) t ( k ) - t ( k - 1 ) (
equation 1 ) ##EQU00003##
where: t(k) is the actual time; t(k-1) is a time prior to the
actual time .theta..sup.i(k) is the elevation angle of the
satellite Si relative to the aircraft at time t(k);
.theta..sup.i(k-1) is the elevation angle of the satellite Si
relative to the aircraft at time t(k-1); and
t set i = .theta. mask - .theta. i ( k ) a + t ( k ) ( equation 2 )
##EQU00004##
where: t.sup.i .sub.set is the setting time of the satellite Si;
.theta.mask is the masking angle selected for the satellite
navigation system to which satellite Si belongs;
[0061] The actual time t(k) corresponds with the time said
satellite set is determined. The preceding time t(k-1) is selected
so as to provide a sufficiently significant difference of the
elevation angle of the satellite in order to obtain sufficient
accuracy of its variation in function of time. As non-limiting
example, t(k-1) can be selected one second prior to t(k).
[0062] FIG. 5 illustrates the notion of the elevation angle
.theta.' of a satellite Si relative to aircraft 1. For this
purpose, a horizontal plane 5 is assumed located at the height of
the aircraft. The x and y axes shown on the figure are included in
this horizontal plane and the z axis is perpendicular to said
plane. A line segment connects aircraft 1 and satellite Si. The
elevation angle .theta.' of the satellite Si relative to the
aircraft is the angle between said line segment and said horizontal
plane 5. The orthogonal projection of said line segment on the
horizontal plane 5 forms and angle .phi. relative to the x axis.
The elevation angle is called sometimes site angle.
[0063] An approximation of the variation .alpha. of the satellite
elevation relative to the aircraft in function of time can be
calculated with equation 1. This variation .alpha. takes into
account both the movement of the satellite and the movement of the
aircraft. It is used in equation 2 to estimate the setting time of
the satellite Si relative to the aircraft. The approximation used
is based on assuming that the variation .alpha. is constant during
the time interval comprised between T.sub.FAF and T.sub.TD.
[0064] The masking angle .theta..sub.mask is the angle below which
it is assumed that on the one hand, there is a risk that the
signals emitted by the satellite will not be received by the
aircraft, for instance due to the presence of obstacles, and on the
other hand, that there is a risk that the measurements are not
sufficiently accurate. In aeronautics, the value of this masking
angle is usually 5 degrees for satellites of the GPS navigation
system and 10 degrees for the satellites of the Galileo system.
[0065] If the estimated setting time t.sup.i .sub.set of the
satellite Si belongs to the time interval [t.sub.FAF; t.sub.TD] it
is assumed that this satellite Si will be setting during said time
interval, therefore during the final approach phase of the
aircraft. Therefore, this satellite Si is added to the set of
satellites that will be excluded from the calculation of the
aircraft position information.
[0066] The above described implementation mode allows for
determining in approximate but simple manner the setting times of
satellites relative to the aircraft during the approach. In
alternative manner, a determination mode can be envisioned based on
calendar or ephemeris type information relative to the different
satellites, for instance contained in a database, which can be used
in combination with information relative to the path of the
aircraft, to calculate the setting times of the different
satellites relative to the aircraft.
[0067] During the final approach, said position information of the
aircraft is determined in iterative manner, by preference in a
position calculation unit 34 of GNSS receiver 30, while excluding
the information coming from the satellites belonging to the set of
satellites determined prior to the start of the final approach. In
this way, a jump is avoided in the value of said position, which
would be due to a satellite setting relative to the aircraft during
final approach, since the satellites will be setting during this
final approach are excluded from the calculation of the aircraft
position. FIG. 4 is a block diagram of GNSS receiver 30. A radio
frequency (RF) receiver head 38 receives high frequency signals
coming from different GNSS satellites (received by means of one or
more antennas). The head amplifies and reduces the frequency of
said signals. The signals exiting said radio frequency receiver
head 38 are processed by a signal processing module 32 which
calculates pseudo distance measurements for each of the GNSS
satellites visible from the aircraft. The position calculation unit
34 receives these pseudo distance measurements and uses them to
determine the position of the aircraft, while excluding the pseudo
distance measurements corresponding with satellites belonging to
said set of satellites determined prior to the start of the final
approach.
[0068] In a preferred mode, are excluded furthermore from said
determination the satellites that will rise relative to the
aircraft, during the final approach. In this way, a jump is avoided
in the value of said position due to the rising of a satellite
relative to the aircraft during the final approach. This preferred
mode is based on freezing, prior to the start of the final
approach, a set of satellites that will be used for determining the
position of the aircraft during the whole final approach.
[0069] The position information determined by the calculation unit
34 of GNSS receiver 30 is used for guiding the aircraft along its
final approach path. To this end, this position information is
transmitted to xLS calculator 15 which determines the deviations of
the aircraft relative to the final approach path. These deviations
are transmitted to the AFS guidance calculator 18 which determines
the guidance instructions of the aircraft. The guidance of the
aircraft according to these guidance instructions is implemented
either through an automatic pilot receiving said guidance
instructions, or through the pilot of the aircraft using a flight
director receiving said guidance instructions.
[0070] The guidance of the aircraft along its final approach path,
in function of said aircraft position information, takes place up
to a decision height corresponding with the anticipated approach.
This guidance can continue until automatic landing in the case of
certain approaches.
[0071] In a particular implementation mode, the determination of
the aircraft position information by the position calculation unit
34 takes into account, not only information coming from GNSS
satellites, but also information sent from an augmentation system
and processed by an integrity monitoring system 36 as shown in FIG.
4. This improves in particular the integrity of said aircraft
position information.
[0072] The used augmentation technique can be a SBAS or ABAS type
of technique, which improves the integrity of said aircraft
position information without requiring a ground station.
Nevertheless, it is also possible to use a GBAS augmentation
technique which relies on ground stations.
[0073] In another particular implementation mode, the satellites
providing the information used for determining said position
information belong to a plurality of GNSS satellite navigation
systems. The GNSS navigation systems likely to be used are for
instance, and in non-limiting manner: GPS (United States), Galileo
(Europe), GLONASS (Russia), Compass/Beidou (China) . . . This
implementation mode offers the advantage of being able to determine
the aircraft position information even if the number of
non-excluded satellites belonging to one constellation of
satellites, of one of the described GNSS navigation systems, is
insufficient to determine said position information with the
required accuracy and integrity. In fact, by combining the
information coming from satellites belonging to several
constellations, corresponding with different GNSS satellite
navigation systems, sufficient measurements of pseudo-distances are
obtained relative to these satellites to determine the aircraft
position information.
* * * * *